1. Field of the Invention
This invention relates to a MEMS (Micro-electromechanical System) variable capacitor having a piezoelectric actuation mechanism based on a piezoelectric thin film.
2. Background Art
In recent years, a variable capacitor and a switch based on an actuator fabricated by MEMS technique have been receiving attention. That is, a movable electrode is formed on a beam supported in air above a substrate, a fixed electrode is formed on a facing substrate, and then the variable capacitor and the switch are constituted so that actuating the beam by an electrostatic force, a thermal stress, an electromagnetic force and a piezoelectric force varies a distance between the movable electrode and the fixed electrode.
Among them, those using a piezoelectric inverse action as an actuating force for the movable beam have a lot of advantages, because they can vary the distance between the movable electrode and the fixed electrode continuously and largely and thus a capacitance variation coefficient can be large. Moreover, air or gas is used as a dielectric material and thus an extremely large Q-value is achieved.
Moreover, these variable capacitor structures are used just as it is and the variable electrode and the fixed electrode are brought together at a distance of an extremely thin dielectric film, and thereby it is also possible to provide the capacitor with the function of a capacitive type switch. Compared with a semiconductor switch, the switch created by this MEMS technique has both a low ON-resistance and a highly insulating separation characteristic at OFF, and thus after all has been receiving attention.
However, the MEMS capacitor based on a piezoelectric actuation mechanism has a long thin beam structure supported in air and including a piezoelectric layer sandwiched between an upper and lower electrodes. Consequently, there is a serious problem that the beam is warped up and down by a small amount of residual stress of materials constituting the beam. Therefore, it is very difficult to create a capacity value of the variable capacitor before/after voltage application as designed.
For example, PZT (lead zirconium titanate) known as a piezoelectric film having large electrostriction effect needs to be formed at room temperature and annealed at about 600° C. in order to a well-defined film quality, but the annealing causes volume contraction and the residual strain inevitably increases.
Conversely, film formation is possible near room temperature, and as for AlN (aluminum nitride) and ZnO (zinc oxide) serving as a piezoelectric film which relatively precise control of the residual stress in the film formation is possible depending on a film formation condition, the electrostriction effect is one order of magnitude smaller than PZT.
Therefore, there is a conflicting problem that materials having a large electrostriction effect is difficult to control the residual strain and materials relatively easy to control the residual strain has a small electrostriction effect. This is one of the largest problem preventing industrial application of a piezoelectric drive MEMS variable capacitor.
Inventors have already invented a piezoelectric actuator having a folded type structure (see JP-A 2006-87231 (Kokai)). This folded type piezoelectric actuator includes a first beam having one end fixed to a substrate, another end serving as a connection end, and a piezoelectric film sandwiched between a pair of electrode films, a second beam having one end serving as a connection end, extending from the connection end in an opposite direction to the first beam, having another end serving as an action end and essentially the same structure and dimensions as the first beam, and a fixed electrode placed on the substrate opposed to the action end. That is, the folded structure is fabricated by placing two beams having the same structure and shape in parallel and connecting end portions mutually, thereby even if beams are warped by the residual stress in the film formation, it becomes possible to cancel the warp, because two beams warp simultaneously.
Inventors have further invented a piezoelectric actuator having a W type folded structure, which two piezoelectric actuators having the folded structure are placed symmetrically about a line and the action ends are mutually connected in order to solve exactly the warp correction effect caused by the folded structure (see JP-A 2008-5642 (Kokai)). They have succeeded in suppressing the warp of the beams to 1% or less by this W type folded structure.
These inventions have almost resolved the warp problem of the piezoelectric actuator, but a portion of the movable electrode is also based on a unimorph structure made of a lower electrode/a piezoelectric film/an upper electrode/a support film similar to the actuator or a bimorph structure made of a lower electrode/a lower piezoelectric film/an upper piezoelectric film/an upper electrode, therefore, there is also a problem that slight residual stress produces a warp on the movable electrode portion and even if the movable electrode contacts the fixed electrode by the piezoelectric actuator, only part of the movable electrode closely contacts and the maximum capacity value is small.
Moreover, there has been a problem that a difference of thermal expansion coefficient between the movable electrode and the substrate produces a difference of dimension between the movable electrode and the substrate, for example, at a high temperature and low temperature, thus a stress of contraction or extension is applied to the movable electrode, the movable electrode is deformed and a contact area between the movable electrode and the fixed electrode is reduced.